A Public Land Mobile Network (PLMN) is a network established and operated by an administration or recognized operating agency (ROA) for the specific purpose of providing land mobile communication services to the public. PLMN provides communication possibilities for mobile users. A PLMN may provide service in one or a combination of frequency bands. A relationship exists between each subscriber and his home PLMN (HPLMN). If communications are handled over another PLMN, this PLMN is referred to as the visited PLMN (VPLMN). A PLMN area is the geographical area in which a PLMN provides communication services according to the specifications to mobile users. In the PLMN area, the mobile users can set up calls to other users of a terminating network. The terminating network may be a fixed network, the same PLMN, another PLMN or other types of PLMN. Terminating network users can also setup calls to the PLMN. The PLMN area is allocated to a PLMN, which is determined by the service provider.
data and internet access. Access to PLMN services is achieved by means of an air interface involving radio communications between mobile phones and base stations with integrated IP network services. One PLMN may include multiple radio access networks (RAN) utilizing different radio access technologies (RAT) for accessing mobile services. A radio access network is part of a mobile communication system, which implements a radio access technology. Conceptually, RAN resides between a mobile device and provides connection with its core network (CN). Depending on the standard, mobile phones and other wireless connected devices are varyingly known as user equipment (UE), terminal equipment, mobile stations (MS), etc. Examples of different RATs include GERAN (GSM) radio access network, UTRAN (UMTS) radio access network, and E-UTRAN (LTE) radio access network.
When a mobile device is turned on, a network selection process starts, whereby candidate PLMNs are chosen, one at a time, for attempted registration. In GSM networks, when an MS is switched on, the International Mobile Subscriber Identity (IMSI) attach procedure is executed. The ISMI attach procedure is required for the MSC and VLR to register the MS in the network. Similarly, in UMTS and LTE networks, when a UE is switched on, it searches for a mobile network to connect. Once the UE selects its desired network, it will try to attach to the network and attempts registration with the network. Typically, UE supports both manual and automatic network selection mechanisms.
To facilitate the network selection process, a UE may have an Operator Controlled PLMN Selector list and a User Controlled PLMN Selector list stored on the SIM/USIM card. Both PLMN Selector lists may contain a list of preferred PLMNs in priority order. A PLMN Selector list, including HPLMN, may have multiple occurrences, with different radio access technology identifies, e.g., E-UTRAN, UTRAN, or GERAN. The UE may utilize all the information stored in the SIM/USIM related to network selection, e.g., HPLMN, Operator Controlled PLMN Selector list, User Controlled PLMN Selector list, and Forbidden PLMN list. If registration on a PLMN is successful, the UE indicates this PLMN (“the registered PLMN”) and be capable of making and receiving calls on it. However, if registration is unsuccessful because the IMSI is unknown in the home network or the UE is illegal, then the UE does not allow any future registration attempts, until the UE is next powered-up or a SIM/USIM is inserted.
When a registration attempt by the UE is rejected by a network with an indication of “permanent” PLMN restriction, the PLMN identity is written to a list of “Forbidden PLMNs” stored in a data field in the SIM/USIM. If a successful registration is achieved on a PLMN in the Forbidden PLMN list, the PLMN is then deleted from the list. The Forbidden PLMN list is used to avoid unnecessary registration attempts. For example, some networks may suffer severe network errors that cannot be resolved in short term. Depending on the underlying cause of the error, it may make no sense to continue to try to attach or register to the network. In that case, continuing to try will wastefully consume UE power and use system resources when there is no chance of success. In addition, some errors occurred in one of the networks may not occur in another network. As a result, reattempt in the network that occurs severe network error will delay the initiation of other recovery mechanism that can successfully restore service to the user. A well-designed forbidden PLMN list is thus desirable.